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DS - inequ

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DS - inequ [#permalink]

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New post 02 Nov 2008, 03:36
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A
B
C
D
E

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guys
can you help me with this ... don´t have an OA!

cheers
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Re: DS - inequ [#permalink]

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New post 02 Nov 2008, 04:17
E.

From stmt1: 7x - 2y > 0, x > 0 and y can be +ve or -ve.
From stmt2, x+y > 0 and again, y can be +ve or -ve.

Combining two also gives x > 0 with y having any values.

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Re: DS - inequ [#permalink]

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New post 02 Nov 2008, 07:05
scthakur wrote:
E.

From stmt1: 7x - 2y > 0, x > 0 and y can be +ve or -ve.
From stmt2, x+y > 0 and again, y can be +ve or -ve.

Combining two also gives x > 0 with y having any values.


can we not do the following ??

Combining the two statements .. we can solve the two inequalities as we solve simultaneous equations

eliminating x
a) 7x-2y >0 (stmt 1)
b) 7x+7y > 0 (stmt 2 * 7)

a-b => -9y > 0 or y<0

hence C

Am I doing anything wrong ?
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Re: DS - inequ [#permalink]

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New post 02 Nov 2008, 07:33
amitdgr wrote:
scthakur wrote:
E.

From stmt1: 7x - 2y > 0, x > 0 and y can be +ve or -ve.
From stmt2, x+y > 0 and again, y can be +ve or -ve.

Combining two also gives x > 0 with y having any values.


can we not do the following ??

Combining the two statements .. we can solve the two inequalities as we solve simultaneous equations

eliminating x
a) 7x-2y >0 (stmt 1)
b) 7x+7y > 0 (stmt 2 * 7)

a-b => -9y > 0 or y<0

hence C

Am I doing anything wrong ?


I'll pick E.

stmt 1: y can be +ve or -ve
stmt 2: Just says -x < y. y can be -5 and x can 10. Or y can be 10 and x can be 5. Insuff

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Re: DS - inequ [#permalink]

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New post 03 Nov 2008, 00:29
amitdgr wrote:
can we not do the following ??

Combining the two statements .. we can solve the two inequalities as we solve simultaneous equations

eliminating x
a) 7x-2y >0 (stmt 1)
b) 7x+7y > 0 (stmt 2 * 7)

a-b => -9y > 0 or y<0

hence C

Am I doing anything wrong ?


Subtracting two inequalities is not correct all the time. For example, 2 > 0 and 3 > 0 but, (2-3) < 0. However, additions is ok.

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Re: DS - inequ [#permalink]

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New post 03 Nov 2008, 02:01
scthakur wrote:
amitdgr wrote:
can we not do the following ??

Combining the two statements .. we can solve the two inequalities as we solve simultaneous equations

eliminating x
a) 7x-2y >0 (stmt 1)
b) 7x+7y > 0 (stmt 2 * 7)

a-b => -9y > 0 or y<0

hence C

Am I doing anything wrong ?


Subtracting two inequalities is not correct all the time. For example, 2 > 0 and 3 > 0 but, (2-3) < 0. However, additions is ok.


My two cents, since we don't know wether y>0 or <0 so we can't do the substraction...as our friend explained using example: 2 > 0 and 3 > 0 but, (2-3) < 0.

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Re: DS - inequ [#permalink]

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New post 03 Nov 2008, 02:37
scthakur wrote:
amitdgr wrote:
can we not do the following ??

Combining the two statements .. we can solve the two inequalities as we solve simultaneous equations

eliminating x
a) 7x-2y >0 (stmt 1)
b) 7x+7y > 0 (stmt 2 * 7)

a-b => -9y > 0 or y<0

hence C

Am I doing anything wrong ?


Subtracting two inequalities is not correct all the time. For example, 2 > 0 and 3 > 0 but, (2-3) < 0. However, additions is ok.


Ok :) I get it ..... Is adding 2 inequalities fine ?
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Re: DS - inequ [#permalink]

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New post 03 Nov 2008, 02:49
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amitdgr wrote:
Ok :) I get it ..... Is adding 2 inequalities fine ?


Addition is the only thing that should be done on inequalities.

For example, a > 0, b > 0 and hence (a+b) > 0.
Alternatively, a < 0, b < 0 and hence (a+b) < 0.

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Re: DS - inequ [#permalink]

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New post 03 Nov 2008, 03:07
scthakur wrote:
amitdgr wrote:
Ok :) I get it ..... Is adding 2 inequalities fine ?


Addition is the only thing that should be done on inequalities.

For example, a > 0, b > 0 and hence (a+b) > 0.
Alternatively, a < 0, b < 0 and hence (a+b) < 0.


Thanks scthakur :) +1
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Re: DS - inequ   [#permalink] 03 Nov 2008, 03:07
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DS - inequ

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